Phase equilibrium in the Gd-Ni-In system at 870 K

1 Introduction

The ternary systems rare earth-transition metal-p element are particularly diverse. Some of these systems show the formation of ternary compounds homogeneously over the whole rare earth (RE) concentration range while others exhibit ternary compounds that are isotypic or isopointal to structures of binary compounds (so-called coloring variants). Many other ternary systems are characterized by the formation of extended solid solutions based on binary compounds.

The systems RE-T-In (T=3d metal) are probably the most unique families. They show extreme variations and are characterized by the formation of intermetallic compounds with complex crystal structures and frequently with unique physical properties. To give an example, the ternary compounds RENi_1−xIn_1+x (x=0–0.25) with ZrNiAl-type structure [1] change the magnetic ground state from ferromagnetic to antiferromagnetic as a function of their composition. Furthermore, Ce₂CoIn₈ and CeCoIn₅ are heavy-fermion superconductors [2], [3]. In total, more than 300 ternary compounds have been discovered and characterized in the RE-3d-metal-In systems [4]. Until recently, the isothermal sections have mainly been constructed for the systems with copper [5] and only few ones have been determined for ternary systems with other 3d metals. Particularly, isothermal sections of the phase diagrams have been constructed for Ce-Cо-In [6], Er-Cо-In [7], partially for the Pr-Co-In system [8], and for {Gd, Tb, Dy}-Fe-In [9], [10]. As for the systems with nickel, which always have intensively been studied, the isothermal sections have been constructed only for five of them, namely Ce-Ni-In [11], Tb-Ni-In [12], Dy-Ni-In [13], Er-Ni-In [14], and Tm-Ni-In [15].

Therefore, the subject of the present work is the investigation of the Gd-Ni-In ternary system, which is part of our systematic phase analytical investigations of the systems rare earth metal-transition metal-indium [4]. A complete isothermal section in the full concentration range at T=870 K has been constructed for the Gd-Ni-In system, with the focus on phases existing with a certain homogeneity range.

The binary systems Gd-Ni [16], Ni-In [17] and Gd-In [18], which are relevant to the investigated ternary system, were studied in quite some detail. The phase diagrams of these systems have been constructed and the crystal structures of the binary compounds determined. Almost all binary compounds in these binary systems are characterized by constant compositions. Exceptions are the binary phases Gd_1+xIn, Ni₁₃In₉, ε(Ni_xIn_1−x), and δ(Ni_xIn_1−x), which form small homogeneity regions.

2 Experimental

Starting materials for the preparation of the samples were ingots of gadolinium, nickel and indium, all with nominal purities better than 99.9%. Seventy-three ternary and two binary samples were synthesized in an arc-furnace on a water-cooled Cu plate under an argon atmosphere (Ti sponge was used as a getter material). The buttons were turned over and re-melted two times to ensure homogeneity. The weight losses were smaller than 1% in all cases. The samples were subsequently sealed in evacuated silica tubes and annealed at T=870 K for 720 h and quenched in cold water without breaking the tubes. The samples were obtained as silvery buttons with metallic lustre and are stable in air.

The phase analysis was performed by X-ray powder diffraction using an Enraf-Nonius Guinier camera (type FR 522) equipped with an imaging plate detector (Fuji film BAS-1800, CuKα₁ radiation) and two different powder diffractometers (DRON-3M and Bruker D8 Advance, CuKα radiation). The indexing of the obtained diffraction data of the ternary samples was performed by comparison with calculated data using the program Powder Cell [19].

The structure of some phases was studied by X-ray powder diffraction. The powder intensity data was collected on an automatic diffractometer STOE STADI P with a linear PSD detector (CuKα₁ radiation, step mode, curved Ge monochromator). The value of the linear absorption coefficient was estimated from the logarithmic ratio between the primary beam intensity and its intensity after passing through the measured samples. The program FullProf [20] was used for the Rietveld refinements of the crystal structures.

The microstructure of the samples was investigated visually on polished and etched surfaces by using a microscope NEOPHOT 30 in reflected light. In selected cases, the composition of selected samples was analyzed in scanning electron microscopes (LEICA 420i or REMMA-102-02) through energy dispersive analyses of X-rays. Selected scanning electron micrographs were taken in the same microscopes.

3 Results and discussion

The isothermal section of the Gd-Ni-In ternary system was established at T=870 K for the whole concentration range, by means of X-ray powder diffraction and partly by EDX analyses (Fig. 1). The existence of the known binary compounds [16], [17], [18] has been confirmed: Gd₂In (Ni₂In type), Gd₅In₃ (W₅Si₃ type), Gd_1+xIn (CsCl type), Gd₃In₅ (Pu₃Pd₅ type), GdIn₃ (AuCu₃ type); Gd₃Ni (Fe₃C type), GdNi (FeB type), GdNi₂ (MgCu₂ type), GdNi₃ (PuNi₃ type), Gd₂Ni₇ (Gd₂Co₇ type), GdNi₅ (CaCu₅ type), Gd₂Ni₁₇ (Th₂Ni₁₇ type), Ni₃In (Ni₃Sn type), Ni₂In (Ni₂In type), phase ζ (Ni_xIn_1−x, NiAs type), Ni₁₃In₉ (Ni₁₃Ga₉ type), NiIn (CoSn type), phase δ (Ni_xIn_1−x, CsCl type), and Ni₂In₃ (Ni₂Al₃ type).

Fig. 1:

The isothermal section of the Gd-Ni-In system at T=870 K.

Earlier, we and other authors found the existence of a number of ternary compounds in the Gd-Ni-In system. Their basic crystallographic parameters are listed in Table 1. Thirteen ternary indides were identified under our experimental conditions (870 K section). Two indides were not found, namely Gd₃Ni_2.26In_3.74 (Lu₃Co₂In₄ type), which has been synthesized by Heying et al. at 1020 K [21], and HT-GdNiIn₂ (own structure type), which has been obtained by Zaremba et al. at 1270 K [22]. The crystal structures have been determined for almost all ternary indides, the compounds ~Gd₆Ni₂In and ~Gd₃Ni_0.05In_0.95 being the exceptions. The last one possibly derives from the binary phase “Gd₃In”, which is stabilized by a small admixture of Ni. Similar cases of stabilization of “Gd₃In” by a transition metal have been observed in previously studied ternary systems, e.g. Gd-Fe-In [10] and Gd-Mn-In [23].

Taking into account the existence of a continuous series of solid solutions between the compounds RNi₂ (MgCu₂ type) [30] and RNi₄In (MgCu₄Sn type) [4] and the solid solutions based on the binary compound NiIn (CoSn type) in related systems with the rare earth elements of the yttrium subgroup, special attention has been paid to these regions of the Gd-Ni-In system. The results of the phase analysis showed that binary GdNi₂ does not dissolve indium and ternary GdNi₄In forms a small homogeneity region along the section of 66.7 at.% Ni. A detailed investigation of the ternary phase with MgCu₄Sn type was performed by X-ray powder diffraction on the Gd₂₀Ni₆₆In₁₄ and Gd₁₇Ni₆₆In₁₇ samples. Figure 2 shows the X-ray diffraction patterns of the respective samples. The crystal structure data and details of the structure refinements are given in Table 2. The homogeneity region of the investigated phase thus extends from 13 to 17 at.% In and the composition is described by the formula Gd_1-1.22Ni₄In_1-0.78.

Fig. 2:

Observed (·), calculated (–) and difference X-ray diffraction patterns of the Gd₂₀Ni₆₆In₁₄ (top – Gd_1.22Ni₄In_0.78 reflections, bottom – GdNi₃ reflections) and Gd₁₇Ni₆₆In₁₇ (top – GdNi₄In reflections, bottom – GdNi₅ reflections) samples.

The samples with the starting compositions Gd₂₉Ni₆₆In₅ and Gd₂₀Ni₆₆In₁₄ were additionally investigated by EDX analysis. The results are shown in Fig. 3. The microprobe analysis results of these samples are in good agreement with the phase analysis results and confirm the absence of a solid solution based on GdNi₂ and the existence of a small homogeneity region for GdNi₄In.

Fig. 3:

Scanning electron micrographs of Gd₂₉Ni₆₆In₅ (top) and Gd₂₀Ni₆₆In₁₄ (bottom) samples.

In order to check the existence of a solid solution based on binary NiIn, a number of samples were synthesized along the section of 50 at.% Ni. The precise phase analysis demonstrated the formation of such a solid solution in the Gd-Ni-In system similar to the other RE-Ni-In systems with Dy, Tb, Er and Tm [12], [13], [14], [15]. Despite the presence of small amounts of impurities in the samples, we could determine the basic crystallographic parameters for this solid solution. Figure 4 shows the X-ray diffraction patterns of the samples. The crystal structure data and details of structure refinements are listed in Table 3. The refinement of the crystal structures of these phases was based on the models of the solid solutions Er_0-0.12NiIn_1-0.89 [14] and Dy_0-0.18NiIn_1-0.95 [13]. In the latter, besides the position of the dysprosium atoms, there is also an additional, partly occupied indium position. The best R-factors were obtained for the structure model of Dy_0-0.18NiIn_1-0.95. The EDX analysis of the Gd₃Ni₅₀In₄₇ sample (Fig. 5) confirmed the formation of the solid solution based on binary NiIn.

Fig. 4:

Observed (·), calculated (–) and difference X-ray diffraction patterns of the Ni₅₀In₅₀, Gd₃Ni₅₀In₄₇ (top – NiIn reflections, bottom – GdNi₄In reflections), Gd₅Ni₅₀In₄₅ (top – NiIn reflections, middle – Ni₂In₃ reflections, bottom – Gd₄Ni₁₁In₂₀ reflections) and Gd₇Ni₅₀In₄₃ (top – NiIn reflections, middle – Gd₄Ni₁₁In₂₀ reflections, bottom – Ni₂In₃ reflections) samples.

Fig. 5:

A scanning electron micrograph of the sample with the initial composition Gd₃Ni₅₀In₄₇.

The data sets suggest that the including-subtraction-type solid solution based on binary NiIn extends up to 7 at.% Gd (Fig. 6) and its composition can be described by the formula Gd_0-0.14NiIn_1-0.98. The inclusion of gadolinium and indium atoms on the site 2e with a simultaneous exclusion of a small amount of indium atoms on 1a takes place in the homogeneity range of this solid solution.

Fig. 6:

Dependence of the lattice parameters and unit cell volume V on the content of Gd in the solid solution Gd_0-0.14NiIn_1-0.98.

The phase relationships in the ternary Gd-Ni-In system are similar to those in the systems RE-T-In with the rare earth metals of the yttrium subgroup. The common feature of all these systems is the existence of a large number of isostructural ternary compounds, the formation of homogeneity ranges for compounds with the ZrNiAl type and including-subtraction-type solid solutions based on the binary compound NiIn. However, also some differences have been observed concerning the region of existence of the RET₄In (MgCu₄Sn type) compounds. While formation of continuous series of solid solutions between RENi₂ (MgCu₂ type) and RENi₄In compounds occurs in the ternary systems with Tb, Dy and Tm, the compound GdNi₄In has a small homogeneity range and GdNi₂ does not dissolve indium. Differences are also noted when comparing the systems Gd-Ni-In and Gd-Cu-In [4]. A much smaller number of ternary compounds (seven) exist in the Gd-Cu-In system. Most of them are isostructural to the respective compounds from the Gd-Ni-In system (compounds with structure types YNi₉In₂, MgCu₄Sn, ZrNiAl, and Mo₂FeB₂).

4 Conclusion

The interaction of indium with gadolinium and nickel has a multifaceted character. Thirteen ternary compounds with complex structures are formed in the Gd-Ni-In system at T=870 K. Two ternary compounds, namely Gd_1-1.22Ni₄In_1-0.78 and GdNi_1.0-0.7In_1.0-1.3 reveal homogeneity ranges through Gd/In and Ni/In substitutions, respectively. The majority of the binary compounds does not dissolve the third component, the exception being binary NiIn, which forms an including-subtraction-type solid solution.